Vacuum circuit breakers can be used at medium voltage level for high current interruption at occasional short circuit current fault and for load current switching (interruption and contacting). A vacuum circuit breaker can include a vacuum chamber in which two contacts (or electrodes) are located that are moved towards each other or away from each other for closing or opening an electrical path in the circuit breaker. When moving the contacts away from each other, a burning arc can arise that has to be extinguished for interrupting the current. For high current interruption, however, vacuum circuit breakers for interrupting currents higher than 50 kA can be a challenge.
In order to achieve high current interruption performances, the erosion of the circuit breaker contacts can be limited, which results from a local overheating from the concentrated burning arc. Hence, the heat arising from the vacuum arc can be managed by spreading out the energy over the whole surface of the contacts. Up to now, at least two methods were used to control the vacuum arc in a way to distribute the heat flow over the contacts area.
The arc control in a vacuum interrupter can be achieved by generating either a transverse magnetic field (TMF) to drive the constricted arc in rotating motion under the effect of Lorentz forces, or an axial magnetic field (AMF) to confine the charged particles around the magnetic flux lines and to stabilize the arc by making it diffuse over the whole contact surface with low current density. An axial direction may be a direction substantially parallel to the movement of the contacts or substantially orthogonal to the facing contact surfaces of the field generating elements. A transversal direction may be a direction substantially orthogonal to the axial direction.
In most designs of AMF based vacuum interrupters, the AMF strength and distribution can be concentrated at the center of interrupter contacts leading to high erosion and interruption failure, for example, at high current. Accordingly, there may be a use for a contact design to prevent the concentration of the AMF in the center of electrodes at high current level.
One solution can be to introduce ferromagnetic element into the contacts assembly, for example, placed at the periphery of the contacts, which can shift the AMF maximum towards the contacts edges.
Another solution can be to introduce further components into the contact assembly which can generate a further AMF in the center of the contacts, which can lower the AMF maximum in the center of the contacts.
For example, US Patent Publication No. 2010/0230388A1 relates to an electrode for a vacuum interrupter. The electrode includes a contact electrode plate, an inner coil electrode and an outer coil electrode. The coil electrodes are formed of an electric conductor having an open loop shape and supporting pins.
However, these solutions may result in contacts with a high resistance that can induce high current losses an excessive thermal heating with the nominal current, and with complicated configurations that may render the manufacturing process slow and difficult and may induce high manufacturing costs.